Steps to reproduce:
1. Create a new project in an empty local PlasticSCM repo
2. Create a Prefab (or import an FBX model)
3. In Version Control choose PlasticSCM (Edit -> Project Settings -> Version Control)
4. Make sure to ignore the asset in question by using ignore.config
5. Checkin .meta file of the asset
6. Go back to Unity to check if you can still edit the properties of the asset

Expected result: The asset properties can still be edited in the Inspector
Actual result: The asset properties are greyed out and not editable

Reproducible with: 2018.4.36f1, 2019.4.28f1, 2020.3.12f1, 2021.1.12f1, 2021.2.0b1

Workaround: Use the Plastic SCM plugin from the Asset Store

Notes:
- So far we know that Prefabs and FBX models are affected, Materials and Playables are unaffected
- In 2018.4 the "Checkout" button at the bottom right of the Inspector, when viewing the asset in question, allows to checkout and edit the asset
- PlasticSCM plugin from the asset store is unaffected
- Version Control package doesn't affect reproduction
- When right-clicking on the affected asset to apply "Check Out" option to it, all of them are also greyed out